The ability to exfoliate single- and few-layer graphene flakes from bulk graphite opened up new avenues into the physics of two-dimensional materials. [1,2] Even though graphene is a good electrical and thermal conductor, its zero band gap property hinders the possibilities in a wide range of potential applications in next generation nanoelectronics and optoelectronics. [3] Moreover, engineering a band gap of graphene makes the fabrication more complicated and reduces the electronic mobility. [4,5] In this regard, molybdenum disulfide (MoS2), a layered transition metal dichalcogenide (TMDC), in which unsaturated d-electron interactions can give rise to new interesting material properties, has garnered a great interest in many next generation nanotechnology applications due to its fascinating electrical, optical and mechanical properties. [6] MoS2, a semiconductor with a finite band gap, is composed of covalently bonded S—Mo—S sheets that are bound by weak van der Waals forces. The band gap of MoS2 can be tuned from direct (˜1.8 eV) [7] to indirect (˜1.0 eV) [8] in its bulk and monolayer forms, respectively. It has been investigated that the band structure and band gap of MoS2 are strongly affected by quantum confinement due to its atomically thin two-dimensional crystal structure. [9]-[11] The band gap of MoS2 can be modified either by reducing the number of layers [9,11-14] or by applying a large local uniaxial strain to the film/membrane [15]. The tunable band gap of the MoS2 makes it promising for applications in optoelectronic devices, such as photodetectots,[16,17] photovoltaics, [18,] photocatalysts and light emitters [19].
An indirect to direct band gap transition from multilayer to monolayer results in pronounced enhancement in photoluminescence (PL) [,9,12] due to a very high quantum yield for monolayer MoS2, which affirms the optical band gap at around 1.9 eV [9,10,12]. While bulk MoS2 has a prominent direct band gap, PL in the bulk is nonexistent owing to excitonic absorption, yet when the direct band gap is dominant, for instance in monolayer regime, direct band radiative recombination becomes the principle method for exciton recombination. [12] It has been found that PL quantum yields for monolayer MoS2 is about 3 orders greater than that of multilayer structure due to radiative recombination across the direct band gap. The PL quantum yield is greatly enhanced when the monolayer MoS2 is suspended. [9] PL of MoS2 is substantially affected by the nature of the substrate/interface, which may have effects on material performance. [20]
The atomically thin two-dimensional structure of MoS2 films/membranes not only opens up new avenues in nanoelectronic and optoelectronic applications but also high surface-to-area ratios. These unique characters make few-layer MoS2 flakes promising sensing devices to many adsorbates. In contrast to brittle bulk phase, mono- and few-layer MoS2 flakes have superior elasticity and flexibility and hence are promising functional membranes. [21] For instance, a laminar separation membrane assembled from atomically thin MoS2 sheets exhibits a water permeance which is 3-5 orders higher than that of graphene oxide membranes. [22]
Bertolazzi et. al recently measured elastic modulus and breaking strength of mono and bilayer MoS2 membranes exfoliated from bulk and transferred onto an array of micro fabricated circular holes in a substrate. [23] According to their measurements, in-plane stiffness of monolayer MoS2 is 180±60 Nm−1 and an effective Young's modulus of 270±100 GPa, which is comparable to that of steel. These unique material properties of mono and multilayer MoS2 sheets might make them suitable for a variety of applications such as reinforcing elements in composites and for fabrication of flexible electronic devices. For example, several groups recently developed flexible field-effect transistors (FETs) based on the large in-plane carrier mobility, robust mechanical properties, flexible and transparent nature, and low power dissipation of mono/few-layer MoS2. [11,24,25] Further, monolayer MoS2 has recently been utilized as a material for microelectromechanical systems (MEMS) and nanoelectromechanical systems (NEMS) devices. [1,9,11,26] Recently, a few research groups demonstrated the use of monolayer and multilayer MoS2 in integrated circuits, although they were only in proof-of-concept stage. [27,28]
MoS2 has recently garnered a lot of interest in biosensing applications due to its two-dimensional crystal structure, electronic properties, tunable band gap, high thermal and chemical stability. [29] Especially, MoS2 has been utilized in electrochemical devices [30-32] and also in field-effect-transistor (FET) devices [33,34] to use as a biosensor for rapid and high-resolution biosensing applications. Ultrathin membranes are ideal candidates for base-resolution nanopore based DNA sequencing applications, because atomically thin membrane can amplify the baseline current and also the amplitude of the transient current drop without increasing the noise level, which results in a great enhancement in signal-to-noise SNR) ratio. In addition to the great enhancement in ionic current amplitude, transverse tunneling current can also be used for high-resolution electronic base detection. [35-37] This goal could be achieved using 2D materials like graphene [38-41] and transition metal dichalcogenides, for example MoS2 [42,43] and boron nitride [44]. Several groups have recently shown high-resolution DNA detection using mono- and few-layer graphene nanopores [38-41], yet the large noise compared to traditional dielectric material based nanopores and the zero band gap property of graphene hinder the development of graphene-based nanopore sensors to achieve base-resolution detection. Even though a finite band gap can be engineered in pristine graphene, this increases the fabrication complexity and reduces the electronic mobility. The greater noise inherent with atomically thick single layer graphene can be reduced by using three-layer thick graphene (about 1 nm), which consequently increases the signal-to-noise ratio. In this perspective, atomically thin (about 0.8 nm) MoS2 is a better alternative for graphene providing a better signal-to-noise ratio while maintaining its atomically thick property for base-resolution DNA detection. Another issue with graphene nanopores is that DNA sticks a lot to the pore wall as well as the surface during the translocation process due the strong π-π interaction between graphene and DNA, which could prove very challenging for nucleobase detection experiments via transverse tunneling current. In contrast, atomically thick MoS2 can be engineered to have either Mo (molybdenum) or S (sulfur) or both Mo and S terminated sheets, which opens up a new avenue for base-resolution detection experiments.
Few-layer or even mono-layer MoS2 flakes can be exfoliated from bulk crystalline material. Such flakes are widely used in research as they possess perfect crystalline structure as well as pristine quality. However, mechanical exfoliation is an extremely low yield process, which in general results in flakes a few micrometers to a few tens of micrometers in size. Therefore, the mechanical exfoliation approach is handicapped with respect to large-scale, high-quality flake fabrication. Chemical exfoliation is also another well-recognized exfoliation approach, which was known well before mechanical exfoliation. [45,46] There are two types of chemical exfoliation approaches, ion intercalation (the Morrison method) [45] and solvent-based exfoliation (the Coleman method) [47]. The Morrison method is handicapped by some major difficulties, such as relatively high temperature requirement (100° C.) and lengthy reaction time (three days), while the Coleman method suffers from low-yield of single-layer sheets and low MoS2 flake concentration in solution. The chemical vapor deposition (CVD) method has gained great interest for fabricating mono- and few-layer MoS2 sheets due to its ease of synthesis and high efficiency, together with its wide tolerance for growth parameters and substrates. [48-54]
In order to investigate the pristine material characterization of MoS2 sheets, it is essential to have less-contaminated freestanding MoS2 sheets over a freestanding window. Further, it is vital to synthesize high quality freestanding MoS2 membranes for use in membrane-based applications such as nanopore devices, selective molecular sieving devices, gas sensors and other semiconductor devices. To date, production of freestanding MoS2 requires transfer of MoS2 sheets to an appropriately perforated substrate, which often introduces contaminants as well as unintentional wrinkles, cracks and tears into the sheets. In addition to the degradation of the quality of the MoS2 sheets, the transfer process is extremely low-yield and is not scalable to a whole wafer such that there is sufficient amount of membrane surface area for use in membrane related experiments.